Electrostatic seismometer



En -nm Nov. 22, 1955 G. PETERSON ELECTROSTATIC SEISMOMETER Original Filed July 1, 1948 2 Sheets-Sheet l H'ys.

Nov. 22, 1955 G. PETERsoN ELECTROSTATIC SEISMOMETER 2 Sheets-Sheet 2 Original Filed July 1, 1948 IN VEN TOR.

GLEN PETERSON "Vczll.

United States Patent 2,724,819 ELucrRoSTATIc sEIsMoMETER Glen Peterson, Tulsa, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Claims. (Cl. 340-17) This invention relates to improvements in seismometers and particularly to those of the electrostatic type.

This application is a division of my copending U. S.

application Serial No. 36,550, filed July 1, 1948, now issued as U. S. Patent No. 2,599,775.

An object of this invention is to provide a seismometer suitable to the direct modulation of a radio frequency carrier wave to provide a radio link between the point of incident seismic energy and the recorder, thereby replacing the cable link and magnetic seismometer therebetween as presently used, and thereby eliminating the well known disadvantages of a cable connection.

Another object of the invention is to provide a seismometer which by comparison with the more usual forms of this apparatus is small in size and light in weight.

A further object is to provide a seismometer of simple mechanical design and thereby adapted to economical manufacture.

A still further object of the invention is to provide a seismometer of improved sensitivity to seismic waves, which characteristic is obtained in part by the use of a specially designed electrostatic pick-up device.

An important object of this invention is `toprovide elecf trostatic seismometer structures in which forwsmall displacements the ratio of variable capacitance to the total capacitance of the seismometer is high.

A further object of the invention is to provide a high impedance seismometer having a time constant suitable to accepted seismic exploration methods.

By using a radio link between the seismometer and the seismograph recorder in place of the usual cable connection there is permitted a re-evaluation of the principles employed in the design and construction of the seismom eter. With systems operated as at present employing long cables to carry the energy from the seismometer to the recording apparatus, a low impedance seismometer capable of feeding energy directly into the cable is generally elements has many advantages as will hereinafter appear,

including that of great sensitivity. In seismic operations desirable. With the system herein disclosed, omitting the earth displacements used for exploratory purposes are frequently of a magnitude as small as 10-8 inches, and a seismometer having the inherent sensitivity to usefully employ such minute vibrations may when properly de signed in accordance with the disclosure herein, be of the electrostatic or capacity type. Electrostatic seismometers of the type herein disclosed when associated with suitable electronic apparatus permits the useful absorption of earth motions of magnitudes as small as 10-8 inches. Motions of such minute magnitude are availed .of in accordance with this invention by making the non 2,724,819 Patented Nov. 22, 1955 variable capacitance of the seismometer as low as possible and its variable capacitance as high as possible whereby maximum sensitivity is obtained.

Other and more detailed objects of the invention will be apparent from the following description of `the embodiments described herein and illustrated in the attached drawings.

In the accompanying drawings- Y Figure l is a top plan view of an electrostatic seismometer in. accordance with the invention herein disclosed;

Figure 2 is a cross-sectional view taken on the line 2 2 of Figure 1;

Figure 3 is a top plan view of a modiiied form of electrostatic seismometer with some parts broken away;

Figure 4 is a vertical, central, cross-sectional view of the structure of Figure 3;4 y

Figure 5 is a cross-sectional view taken on the line 5 5 of Figure 6 of still another form of electrostaticseis-` mometer in accordance with this invention;

Figure 6 is a side elevational view of this modication;

Figures 7 and 8 are elevational views of modied forms of movable dielectric members for the structures of Figures 5 and 6; t

Figure V9 is a cross-sectional view taken on the line 9 9 of Figure l0 of a modified form of electrostatic seismometer in accordance with this invention showing it immersed in a iiuid;

Figure 10 is a top plan view of the seismometer of the combination of Figure 9 with the fluid housing removed and a part broken away;

Figure ll is a cross-sectional View `taken on the line 11 11 of Figure 9; v

Figure l2 is a detailed plan view of one of the supporting 'springs for the movable vdielectric member of this form of seismometer:

Figure 13 is a front elevational view of the movable dielectric member of this seismometer;

Figure 14 is an end elevational view of `a still further modified form of electrostatic;seismometer immersed in a fluid and showing the housing in cross-section; and

Figure l5 is a detailedielevational View ofthe movable dielectric member of the seismometer of Figure 14.

Illustrated in Figures l and 2 is one form of electrostatic seismometer of sensitivity suitable to seismic exploration. As shown, it comprises a pair of insulating rings 1 and 2 clamping therebetween by means of the rivets or screws 3, a pair of insulating discs 4 and 5 which may be made of some material suitable to the purpose such as mica, cellophane, cellulose acetate and other plastic substances having the required electrical and physical properties. The discs 4 and 5 are respectively coated on their upper surfaces with the silver layers 6 and 7 respectively, which layers may be applied by any known method, but preferably by electrode deposition. interposed between the composite discs 4 6 and 5 7 are the shims or wash` ers 8 which predetermined `the Anormal static spacing between the discs. While the silver coatings 6 and 7 have been shown on` the upper faces of the discs 4 and 5, it will be understood that in some cases the coating may be on the lower face of disc 4. The disadvantage of this, however, is that under more intense vibration than would normally be expected the two 'coatings 6 and 7 might short circuit and interfere with the proper operation of the recording apparatus. The two silver coatings 6 and 7 extend radially as a part of the tabs 10 to make provision for electrical circuit connection thereto. As illustrated, the upper composite disc 4 6 has a central opening to provide for the movement therein of the head of a rivet or other attaching means to which a weight 9 of suitable yproportions is suspended from the lower disc 5 7. As

electronic engineers will appreciate, the coatings 6 and 7 must'be insulatedfrom each other, and therefore in addition to using insulating shims 8 the rivets 3 must be insulated from the coatings as by providing them with an insulating sleeve 3.

`By means of this construction there is provided the .advantage that the metallclms 6 and 7 maybe brought very close together,`increasing the sensitivity of the device butvpreventing short circuit-thereof by contact of onegcoating with the other. `By means of this construction it is possible to separate the two discs by a distanceas small as 0.001 inch, that is the thickness of the shim'S. By making the insulating disc S of very thin material it is possible to provide a total separation between the condenser plates 6 and 7, of as little as 0.0033 inch,so that a displacement of 10-8 inches will produce a variation in capacity of about 3 parts in a million. This variation is sufiicient to effect phase or 'frequency modulation of a high frequency oscillator.

VIt can be shown that the instantaneous .voltage developed bya capacitative seismometer such as that of Figures v 1 and2 is given by the relation Co `da:

'where E isthe bias E. M. F. in volts applied to it, R is the load resistance in ohms, Co is the initial-capacitance of the seismometer in farads, Xo is the initial separation of `theplates ofthe seismometer in inches and a dt `is therate of change of the plate Aseparation measured in 'inches per second. Thus if kthese values are E=300 volts, R=108 ohms, Co=l00 M. M. F., Xo=0.003 inch,

Q 10-s inches dt 10*2 seconds then ==10a volts which is the ylimiting value ofthe voltage developed for the given structure. This compares very favorably with about 10'-5 volts for low impedancemagnetic types of seismometers as now commonly used.

A somewhat more instructive relation than the equation given above can l.be drawn if one considers the electrostatic seismometer to becomposed of two capacitors C and ACzconnected in parallel 'through a switch, `and this combination .connectedzin yseries with a battery'E and ya =10 inches per second lresistor R, where AC isthatportion of the-total capacivtance Lthat is variable `with Iground-motion. r Under 4these circumstances, it can vbe shown thatlthe transient pulses of voltage developed by :the seismometerihave anampli- Y tude n dt Iof Equation l could be as large as possible ,in comparison with Xo.

In the form of electrostatic or capacitive seismometers shown in Figures 3 and 4 the variable portion or AC is further increased vin comparison `with *the 'iixe'd 'portion `C.

-ing bushings.

As illustrated, the device-consists of a set of fixed parallel circular plates 12 separated by spacing rings 11 which are of conducting material, which set includes the terminal plates 14 and 15 spaced from the adjacent fixed plates 12 by means of spacer rings 1 1 which are of insulating material. All of these plates and rings are secured together by a series of rivets orfbolts 13 which are conductively in contact withfthe plates 12 but are insulated from the plates 14 and 15 by means of bushing 13 which may be integral with the insulating rings 11 or may be separate insulat- The heads of the rivets 13 are insulated from the plates 14 and l15 by means of insulating washers 13" as shown. The fixed plates 12 are provided with axially aligned concentric openings in which a Set of movable plates 17 are .mounted so that their peripheral edges are slightly spaced radially from the annular Walls defining the openings in the viixed plates 12. The plates '17 are separated by conducting spacers 16 which are ysecured together and to the endplates 14 .and v15 by means 'of a riveting or fastening device 1S which also serves to attach to the resulting movable system aproperly proportioned weighting member 19. The terminal spacers 16 are lindicated as'being of insulating material but they too can be of conducting 4material as the plates 14 and 15 are insulated as previously described from the fixed plates 12. The plate 14 is provided with a terminal connecting tab vv20 and one ofthe plates 12 is provided with a similar tab20 so-that the plate groups 12 and 17 are electrically connected in groups of Which'the plate group 17 is relatively movable withrespect to the plate group 12 under the control of the spring action produced by the end plates 14 and 15. It will be understood that the movable plates V17 are electrically connected to the fastening device 18 andto the spring mounting plates V141 and 15. In normal relaxed position the movable plates 17 lie in the planes of the Xed plates 12, as shown inFigure 4. Thusthe plates of the movable set are placed edge to edge with vthe plates of the fixed set so as to get the greatest change in capacity for given'relative movements betweenthe two sets of plates. This,.of course, `is in comparison with the normal xed capacity C of lthe combination. As a result in Equation A2 is .made as large as possible. To further increasethis factor of the equation the plates 12 and 17 are made very thin.

Inthe case of both structures the weights 9 and 19 are so selected as the particular characteristics of each assembly requires, so that the moving portion thereof is given ,the desired period of motion With respect .to the other.

The construction of Figures 5 to 8 inclusive has the general electrical characteristics explained above as desirable fora capacitive seismometer ybut structural ditferences from the modifications previously described. In this case-there is ,provided a pair of fixed conductive members 21 and .22 which .for the 4sake Vofsimplication are shown in cross-section as E-shaped. Attached to the opposite ends thereof are the pairs of insulating strips or blocks..23 .and -24 and the vspring supporting members 26 Aand 27. The conductor members .21 and 22, the spacers ,23 and 24, andthe spring supporting members 26 and.27. are secured together in stacked and spaced relation as-clearly shownin Figure, by means of rivets yor bolts y,25. .Included in each .stack .are the terminal tabs 29.

The spring members 27 which may .be constructed like that illustratedfin Figure 12, ,or maybe simplyvcontinuous thin lstrips of spring metalserve to supportbetween them a movable memberconsisting of aplate of Vdielectric material composed `of alternate sections 28 .and y28-of different dielectric y properties.

Preferably Athe -sections V28 should havea higher dielectric .constant .than `the Vsections '28. It will be appreciated that when this structure is wana-iv kbA-2 @man subjected to vibrations thehsuspended element will oscillate in a vertical plane to vary4 the dielectric characteristics of the gap between the arms ofthe conducting'E members 21 and 22 and thereby change the capacity of the condenser that is thus formed by the conductor members 21 and 22. The suspended element 28 is a rectangular plate built up of alternate laminations of strips 28 and 28 as will be understoodfrom this description. v

The movable member mayhoweve1", take the forms illustrated in Figures 7 and 8. The structure of `Figure`7 consists of a member 28aof the configuration shown and composed of a dielectric material of uniform dielectric constant throughout its structure. It will be 'supported between the springs 26 and 27 in the same manner as in the member 28, with the result that the cross arms will take the positions in the view of Figure 6, that the bars 2S have. In this case, therefore, with the exception of the vertical bar of the member 28a which connects the horizontal bars, the gaps between the arms of the conducting members 21 and 22 will be composedl partially by air and partiallyby the dielectric material of thehorizontal arms. Thus in effect the member 28a will have the characteristics of the member28 of creating a condition of relatively high and low dielectric capacity so that vertical vibration thereof will cause a similar change in the variable component of the capacity of the condenser structure. To put it more concisely, `with the-structure of Figure 7 air hasbeen substituted as the dielectric *material for that of the bars 28 except at the center of the member 28a at which the vertically connected bar lies. In the structure of Figure 8 the features of the two previous movable members are combined. In the structure of Figure 8 the horizontal bars 28c are made of lmaterial of a higher dielectric constant than the material of the vertical bar 28h, thus in part at least the characteristics of both of the previous structures are provided by the structure of Figure 8.

It will be understood, as is clear from the other forms of the device disclosed, that the number of projections on the stator members 21 and 22 and the cooperating arms on the armature 28 may be increased with due regard to the requirement in accordance with this invention that the variable factor of the total capacitance of the device shall be large with respect to the ixed capacitance thereof.

A still further form of structure is illustrated in Figures 9 to 13 inclusive. In this case, as before, a pair of xed E-shaped conducting members 21 and 22 are placed in opposed relation and assembled in that relation by means of the insulating spacers 32 which are of U-shaped form, as shown in the various iigures, and provided with seats for the conducting members 21 and 22. These parts are secured together to hold the members 21 and 22 in parallel spaced relation and to secure the springs 33 thereto. The springs 33 are provided with struck-out fingers 34 which are secured to the opposite ends of the member 28d. This member, as clearly shown in Figure 13, consists of a slight rectangular plate of insulating material having a low dielectric constant and provided with a series of rectangular windows or openings 28e aligned with respect to the arms of the conducting members 21 and 22 so that the material of the member partly overlaps the aligned areas of these arms and the remaining areas are separated by the air spaces provided by the windows 28e. The terminal tabs 29 are respectively connected to a pair of lead-in wires 29' which are mounted in a suitable housing 30 containing a fluid 31 of high dielectric constant, many of which are well known and suitable for the purpose. In the form illustrated the housing 30 being of insulating material, the lead-in wires 29 can be directly mounted therein. If the housing is of conducting material then suitable insulators for the lead-in wires 29' will be provided.

With this arrangement the spring fingers 34 provide greater vertical flexibility than would be provided by the 6 arrangement of Figure 6, where the `spring members 26 and'27 are continuous strips of spring metal.

It will be apparent that the moving portions of the structure of Figures 6 and 9 can be provided with weights as before to adjust the period of vibration thereof. It will also be understood that the structure of Figure 9 will be provided with suitable vent openings so as to permit the proper movement of the insulating fluid 31 as the member vibrates, although in view of the very small movements of the member the openings around the spring fingers 34 will normally be adequate.

A preferred form of electrostatic or capacitive seismometer is illustrated in Figures 14 and 15. It consists of a housing 3S of similar material containing a uid 36 of a high dielectric constant in which the seismometer elements are immersed. These elements consist of a pair of grids 37 and 38 which are supported from a wall of the housing by means of rods or wires 39, 40, 42 and 43 so that these grids are supported from both ends and in parallel spaced relation, as shown. The spacing at the lower ends is further maintained by means of yokes 41 at each side of the grids. The supporting Wires or rods 39, 40, 42 and 43 also provided suitable circuit connection leads for the device which pass to the exterior of the housing directly therethrough if it is of insulating material or at the insulating and Welling bushings (not shown) if it is of conductive material.

Suspended between the grids 37 and 3S is the movable grid 44 corresponding to the moving element of the other devices and suspended by four springs 45 and 46 (see Fig. 15). Attached to the movable grid at the bottom is a suitable weight 47 to impart the proper periodicity to the Each of the grids 37, 38 and 44 is constructed like that illustrated in Figure l5 and each comprises a pair of side bars like the bars 44a connected by a series of parallel cross bars 44h. All of the bars of all of the grids are, of course, made of metal and the horizontal bars can, for example, be welded to the vertical bars. While the springs 45 and 46 may be made of metal so as to provide a conductive connection through the supporting element 44', they are preferably made of glass bers in which case a suitable ilexible jumper connection from the movable grid 44 to the supporting element 44 will be provided. The element 44 is insulated from the housing 35 if it is of metal. This type of structure has the virtue of lightness and the grids can be relatively arranged so that the non-variable portion of the capacity is kept relatively low.

The grid structure illustrated in Figures 14 and l5 is particularly adapted to accomplish the object of this invention because the movable grid can be made very light so as to be very sensitive to small earth movements and the ratio of variable capacitance to fixed capacitance can readily be made large.

From the above description it will be apparent to those skilled in the art that the subject matter of this invention is capable of considerable variation, and I do not, therefore, desire to be strictly limited to the disclosure which has been given for illustrative purposes, but rather by the claims granted me.

What is claimed is:

1. A capacitive seismometer comprising a pair of spaced members, each of said members having at least one extension of electrically conductive material oppositely spaced from a corresponding extension of the other said member; a movable member comprising alternate sections of materials of different dielectric constants positioned between said extensions; and means for resiliently supporting said movable member substantially in alignment with said extensions.

2. The combination in accordance with claim 1 wherein at least one of said alternate sections of said movable member is an opening in said member.

3. The combination in accordance with claim 1 wherein each alternate section comprises a solid material.

4. The combination in accordance with claim Lfurther comprising a housing enclosing said members vand an insulating liquid -Within said housing in which said `members are immersed.

5. A capacitive seismometer comprising a pair .of spaced members, each of said members having agrid comprising spaced bars yof conducting material extending therefrom so that said grids are oppositely spaced vfrom one another; a movable grid comprising spaced barsmof electrically conductive material, the spaces between .the bars of said movable grid being of diierent dielectric material than the bars of said movable grid; and means for resiliently supporting said movable grid between said first and second mentioned grids in alignment therewith.

6. The combination in accordance with claim 5 further `comprising a weight attached to said movable` grid .t

predetermine its periodicity of vibration.

7. The combination in accordance withclaim further comprising a housing enclosing said grids and a dielectric uid in said housing in which said grids are positioned.

8. A capactive seismometer comprising a pair ofspaced conducting members, each of Msaid members having at least one extension oppositely spaced from another extension of the other said member; a movable member comprising a plate consisting of alternate sections of material of different dielectric constants lying between said extensions; and means for resiliently supporting said movable member.

9. A capactive seismometer comprising a pair of spaced conducting members, each of said members having at least one extension oppositely spaced frorn,another exten sion of the other said member; a movable member comprising a substantially vertical bar with `cross arms atlashed-...thereto ,flying between said extensions, Said -bar and saidcmsstarms being ccmpgsed ofdieletric material of nnicrm dielectric constant, the spaces between said cross arms beingnidferent dielectric Nmaterial than said ,cross arms; ,and `means 'for resiliently supporting said movablemember.

l0. A capacitive seismometer comprising a pair .of spd, .Q0nduc1ing members, each of said members having atleast one extension .oppositely spaced from another extension of the other said member; a movable member comprising ,a substantially vertical bar with cross arms attached ktl'leJeto lyingbetween said extensions, said bar and said ycross arms being composed of dielectric mater-ialshavng diieljent dielectric constants; and means for resiliently fsupportingsaid `movable member.

`-References Ctedringthe le of this patent UNITED STATES PATENTS ,2,156,056 Hornung Apr. 25, 1939 2,305,355 Lips et al. Dec. 15, 1942 2,367,017 Gardiner Jan. 9, 1945 2,368,278 -Warshaw Jan. 30, 1945 2,430,757 .Conrad et a1 Nov. 11, 1947 `2,469,137 Strong May 3, 1949 2,476,410 Gardiner July 19, 1949 2,'571-,5071 Welch Oct..16, 1951 2,599,775 Peterson June 10, 1952 FOREIGN PATENTS 132,991 Australia June 1, 1949 509,783 4Great Britain July 20, 1939 l,5l-1,042 yGreat Britain Aug. 9, 1939 

1. A CAPACITIVE SEISMOMETER COMPRISING A PAIR OF SPACED MEMBERS, EACH OF SAID MEMBERS HAVING AT LEAST ONE EXTENSION OF ELECTRICALLY CONDUCTIVE MATERIAL OPPOSITELY SPACED FROM A CORRESPONDING EXTENSION OF THE OTHER SAID MEMBER; A MOVABLE MEMBER COMPRISING ALTERNATE SECTIONS OF MATERIALS OF DIFFERENT DIELECTRIC CONSTANTS POSITIONED BETWEEN SAID EXTENSIONS; AND MEANS FOR RESILIENTLY SUPPORTING SAID MOVABLE MEMBER SUBSTANTIALLY IN ALIGNMENT WITH SAID EXTENSIONS. 